1. Field of the Invention
The present invention relates to a heating element CVD system in which a heating element kept at a specified temperature is disposed in a vacuum chamber (processing container) and in which a raw material gas is decomposed and/or activated by the above-mentioned heating element to deposit a thin film on a substrate placed in the vacuum chamber (processing container) and to a structure for connecting a heating element to an electric power supply mechanism in a heating element CVD system.
2. Description of the Related Art
In manufacturing various kinds of semiconductor devices including an LSI (large scale integrated circuit), an LCD (liquid crystal display) and the like, CVD (chemical vapor deposition) methods have been widely used for forming a predetermined thin film on a substrate.
For example, in a plasma CVD method, a raw material gas is decomposed and/or activated in discharged plasma to form a thin film. In a thermal CVD method, a substrate is heated to induce a chemical reaction to form a thin film, and so on. In addition, there exists a CVD method in which a raw material gas is decomposed and/or activated by a heating element kept at a predetermined high temperature to form a thin film (hereinafter referred to as a heating element CVD method).
In a film forming system for performing the heating element CVD method (hereinafter referred to as a heating element CVD system), a heating element made of a refractory metal such as tungsten or the like is disposed in a processing chamber which can be evacuated to a vacuum. A raw material gas is introduced into the evacuated processing chamber while the heating element is kept at high temperatures from about 1000° C. to 2000° C.
The introduced raw material gas is decomposed and/or activated when it passes over the surface of the heating element. Then, the decomposed and/or activated raw material gas reaches a substrate to deposit a thin film of the material, which is a final objective material, on the surface of the substrate. In this connection, in the heating element CVD methods, a CVD method using a wire-shaped heating element is called as a Hot Wire CVD method, and a CVD method utilizing a catalytic reaction of a heating element for decomposing and/or activating the raw material gas by the heating element is called a catalytic CVD (or Cat-CVD) method.
In the heating element CVD method, the raw material gas is decomposed and/or activated when it passes over the surface of the heating element. For this reason, this method has an advantage of reducing the temperature of the substrate as compared with a thermal CVD method in which reaction is induced only by the heat of the substrate. Further, in the heating element CVD method, plasma is not produced, as it is produced in the plasma CVD method. For this reason, there is no worry that plasma causes damage to the substrate. Accordingly, the heating element CVD method is thought to be a promising candidate as a film forming method for a semiconductor device, a display device and the like of the next generation in which high integration and high functionality have been increasingly developed.
FIG. 7 shows a conceptional view of a conventional heating element CVD system. In a processing container 1, a predetermined processing of forming a thin film is performed on a substrate (not shown). An evacuation system 11 for evacuating the processing container 1 to a vacuum and a specified raw material gas supply system 21 for supplying a raw material gas into the processing container 1 for forming a thin film are connected to the processing container 1. In the processing container 1, a heating element 3 is disposed such that the raw material gas supplied into the processing container 1 passes over the surface of it. An electric power supply mechanism 30 for supplying electric power is connected to the heating element 3, thereby the heating element 3 is heated and kept at a predetermined temperature (a high temperature of about 1600° C. to 2000° C.) required for the heating element CVD method. Further, in the processing container 1, a gas supply unit 2 is arranged in a manner opposite to the heating element 3.
Further, in the processing container 1, a predetermined thin film is formed on the substrate (not shown) by the raw material gas decomposed and/or activated by the heating element 3 that is kept at the predetermined high temperature described above. For this reason, in the processing container 1, a substrate holder 4 is provided for holding the above-mentioned substrate (not shown).
In FIG. 7, it is a gate valve for carrying the substrate into or out of the processing container 1 that is denoted by a reference character 5. Further, the substrate holder 4 is provided with, as is conventionally known, a heating mechanism for heating the substrate, but this heating mechanism will not be shown or described because it is not important in the present invention.
In this connection, in the embodiment shown in FIG. 7, although it is not shown, the raw material gas supply system 21 includes a cylinder filled with the raw material gas, a supply pressure regulator, a flow regulator, a supply/stop switching valve, and the like. The raw material gas is supplied by the raw material gas supply system 21 into the processing container 1 via the gas supply unit 2 provided in the processing container 1.
Further, in a process using two or more kinds of raw material gases, the raw material gas supply systems 21 of the number equal to the number of kinds of gases used are connected in parallel to the gas supply unit 2.
The gas supply unit 2, as described above, is arranged in face with the heating element 3 in the processing container 1. Further, the gas supply unit 2 has a hollow structure and many gas blowing holes 210 on a surface facing the substrate holder 4.
On the other hand, the evacuation system 11 is connected to the processing container 1 via a main valve 12 having an evacuation speed regulating function. The pressure in the processing container 1 is controlled by this evacuation speed regulating function.
In the heating element CVD method, the substrate (not shown) is a substance to be subjected to a predetermined processing of forming a thin film. This substrate (not shown) is carried in and out of the processing container 1 via the gate valve 5.
The above-mentioned heating element 3 is generally formed of a wire-shaped member and is bent in the shape of sawtooth and is held by a support body 31, the surface of which is at least made of an insulator. Further, a power supply line 32 from the electric power supply mechanism 30 is connected to the heating element 3 by a connection terminal 33. An electric power is supplied to the heating element 3 via this connection terminal 33 to heat the heating element 3 to the predetermined temperature required for the heating element CVD method and to keep it at the predetermined temperature.
Usually, a direct current power source or an alternating current power source is used as the electric power supply mechanism 30. The heating element 3 is supplied with electric power from the power source and is set at the predetermined temperature by the passage of the electric current. By heating the heating element 3 to a high temperature, the raw material gas is decomposed and/or activated to effectively form a thin film.
Usually, the heating element 3 is heated to a predetermined temperature (usually, in a film forming process, a high temperature of about 1600° C. to 2000° C.) by the passage of the electric current, so a refractory metal is used as the material for the heating element 3 and, in general, tungsten is used.
A case where a silicon film is formed and a case where a silicon nitride film is formed will be described as examples of forming a thin film by the heating element CVD system shown in FIG. 7. The processes are proceeded as follows.
First, a mixed gas of silane (SiH4) and hydrogen (H2) is used in the case where the silicon film is formed. A mixed gas of silane and ammonia (NH3) is used in the case where the silicon nitride film is formed. The pressure in the processing container 1 is about from 0.1 Pa to 100 Pa. In both cases, the temperature of the heating element 3 is set at a predetermined temperature (usually, a high temperature of about from 1600° C. to 2000° C. in a film forming process), and the temperature of the substrate (not shown) held by the substrate holder 4 is set at a temperature of about from 200° C. to 500° C. by a heating mechanism (not shown) in the substrate holder 4.
In the case where the silicon film or the silicon nitride film is formed under predetermined film forming conditions by the use of a conventional heating element CVD system described above, the following phenomenon is produced. A refractory metal used for the heating element, for example, a tungsten wire described above or the like sometimes reacts with the silane gas to form a silicon compound (hereinafter referred to silicide formation).
A silicide formation mentioned above proceeds from near a connection terminal 33 that is the connection part by which electric power is supplied from the electric power supply mechanism 30 (that is, the connection region of the heating element 3). In the above-mentioned connection region of the heating element 3, the temperature of the heating element 3 becomes lower than 1600° C. at the film forming process. Further, in the above-mentioned connection region of the heating element 3, the reaction speed of the raw material gas with the heating element 3 is faster than the desorption speed of decomposed and/or activated gas species of the raw material gas and raw material gas itself.
The above-mentioned silicide formation changes the composition and the diameter of the heating element 3 and reduces the resistance thereof. As a result, the heating power is reduced and the whole heating element is finally deteriorated. Also, it reduces a film forming speed as hours of use of the heating element is elongated. Further, since the products of the silicide and the like have high vapor pressures in general, they contaminate the deposited film and degrade the quality of the silicon film formed or the silicon nitride film formed as the deterioration of heating element is proceeding.
Therefore, it is necessary to break the vacuum in the processing container 1 to the atmospheric pressure and to change the heating element 3 when a predetermined number of substrates are processed. This change of the heating element 3 results in a problem in productivity.
FIG. 8 is a view showing a part of a support body 31 of a conventional embodiment. In the part of the support body 31 of the conventional embodiment, the heating element 3 is supported by the support body 31 by means of a wire 34 (usually, made of molybdenum) to reduce a contact area of the heating element 3 thereby reducing thermal conduction. The conventional embodiment shown in FIG. 8 is intending to prevent silicide formation, which proceeds from the part of the heating element 3 where its temperature is slightly low.
However, even in this method, the temperature of the heating element 3 at the part in contact with the wire 34 drops inevitably, so that the silicide formation at and from the above-mentioned portion is caused, depending on film forming conditions such as a high silane gas pressure in case of forming the silicon film or the like.
Further, even in this method, it is impossible to eliminate connection to the electric power supply line 32, so that the silicide formation is also caused at the part of the connection terminal 33 as is the case shown in FIG. 7. Therefore, even in a heating element CVD system adopting the constitution shown in FIG. 8, it is necessary to break the vacuum in the processing container 1 to the atmospheric pressure and to change the heating element 3 when a predetermined number of substrates are processed. The change of the heating element 3 results in a problem in productivity.
On the other hand, if films are repeatedly formed in the heating element CVD system, the films are deposited also on the inside of the processing container and are peeled off and results in the cause of the particulate problem. The inventors of the present application proposed a method of effectively removing a film deposited on the inside of a processing container, which becomes an origin of particulates, and an in situ cleaning method of a heating element CVD system (Japanese Patent Application Laid-Open (JP-A) No. 2001-49436).
According to this disclosed method, the gas supply unit 2 of the conventional heating element CVD system shown in FIG. 7 is provided with a cleaning gas supply system having the same constitution as the raw material gas supply system 21, and when cleaning the system, instead of a raw material gas used in forming a film, a cleaning gas is introduced into the processing container 1 via the gas supply unit 2. That is, after the processing container 1 is evacuated, a heating element 3 disposed in the processing container 1 is heated to and kept at a temperature of 2000° C. or more, and a cleaning gas which is decomposed and or activated by the heating element 3 to produce activated species which in turn react with a deposited film to change it into a gaseous substance is introduced into the processing container 1, and the produced gaseous substance is exhausted from the processing container 1 to remove the deposited film from the inside surface of processing container. This method has been made based on findings that when the heating element 3 is kept at a temperature of 2000° C. or more, the heating element 3 itself does not react with the cleaning gas but remains stable.
However, after the above-mentioned method was made, it turned out that even if it was tried to keep the heating element 3 at a temperature of 2000° C. or more, a part near the connection terminal 33, which is the connection part of electric power supply from the electric power supply mechanism 30 to the heating element 3, became low in temperature, and that as the deposited film was being removed, the part was etched by the cleaning gas and was gradually reduced in diameter and finally broken by the reaction of the said part with the cleaning gas. Thus, it is necessary to replace the heating element at a certain time, which results in a problem in productivity.
Further, it was found that in the case where a film was deposited on a large-area substrate of over 1 m by the use of the heating element CVD system shown in FIGS. 7 and 8, there was a room for improvement in the uniformity of thickness of a thin film deposited.
To be more specific, in the Cat-CVD method, in the case where in order to form a film on a large-area substrate by the use of the heating element CVD system shown in FIGS. 7 and 8, a conventional technique is adopted in which a sawtooth heating element 3 droops due to a large-size support frame as large as the substrate, there is presented a problem that the heating element 3 is drooped by thermal expansion. That is, since the sawtooth heating element 3 is thermally expanded by about 1% when it is heated to 1800° C., if the heating element 3 having a length of 1 m is used to form a film on the large-area substrate, the heating element 3 is drooped by 70 mm at the maximum by a thermal expansion of 1%. In the worst case, it is estimated that the heating element 3 is drooped more than the distance between the substrate and the heating element 3, which is usually set at about 50 mm. According to the inventor study, it is found that the gap (distance) between the heated heating element 3 and the substrate subjected to a film forming process greatly affects the uniformity of a film thickness when the film is formed.
At present, it is anticipated that the size of a next-generation glass substrate will be larger than 1 m. For example, it is planned to use a large substrate of 1100 mm by 1250 mm for an LCD and 900 mm by 455 mm for a solar cell. In order to form a film on such a large-area substrate, it is necessary to reduce the degree of drooping of the heating element 3 caused by the thermal expansion mentioned above and to ensure the uniformity of thickness of the film formed on the large-area substrate, so that the present applicant already proposed an improved heating element CVD system (International Publication No. 02/25712 Pamphlet=U.S. Pat. No. 6,593,548 B2.)
This improved heating element CVD system makes it possible to prevent the heating element from being degraded by the raw material gas in a connection region of the heating element connected to the electric power supply mechanism in the heating element CVD system in which the raw material gas introduced into the processing container (vacuum chamber) is decomposed and/or activated by the heating element to deposit the thin film on the substrate arranged in the processing container (vacuum chamber). Also, this improved heating element CVD system makes it possible to prevent the heating element from reacting with the cleaning gas at the time of cleaning for removing the deposited film on the inside of the processing container in the connection region of the heating element connected to the electric power supply mechanism in the heating element CVD system. Then, this makes it possible to provide a heating element CVD system excellent in mass production in which the life of the heating element can be elongated and in which a film forming environment can be stabilized.
Further, this makes it possible to respond to forming a film on a large-area substrate more than 1 m and to provide a heating element CVD system that can ensure the uniformity of the film thickness even in a case where the film is formed on such a large-area substrate.